Diesel Vehicle Pollution Rises At Low Temperatures; On Purpose

Euro 5 and Euro 6 vehicles protect their engines at lower, ambient, temperatures at the expense of the environment.

Real world diesel emission test results

Diesel vehicles deliberately switch off pollution control systems at low temperatures to cut the risk of breakdown, Emissions Analytics suggested. The independent test house measures fuel consumption and emissions in the real world, rather than labs. Its data comes via instruments stowed at the rear of the test subjects.

Emissions Analytics tested 213 models from 31 brands. Euro 5 vehicles – Euro 5 being the emission standards that had to be met between January 2011 and September 2015 – performed particularly poorly. For comparison, on average, nitrogen oxides emissions were 3.6 times beyond the limit when the temperature hit 18 C (or over).

However, as the temperature fell emissions rose to 4.6 times above the limit. Euro 6 models performed better, but were still more polluting than motorists might fairly expect. Nitrogen oxides emissions were 2.9 times above the limit in hot weather, for example. This rose to 4.2 times as the temperature fell below 18 C.

Switching off emission controls is legal

Despite the environmental consequences, manufacturers are allowed to switch off emission control systems in some circumstances. The Society of Motor Manufacturers & Traders - which represents car makes - told the BBC: “The temporary reduction, or switching off, of some emissions control systems under certain temperatures is allowed by law and necessary to avoid damage to vehicles’ engines”.

The spokesperson continued: "Without it, there could be a significant cost to the consumer for major repair work. In its recent report, government recognised the need for such technology and was satisfied with how vehicle manufacturers were using it.”

“Manufacturers are investing billions in developing ever-more advanced technology. This, along with new real driving emissions regulation from next year, will see significant lowering of emissions across a full range of driving conditions and temperatures.”

Mr Molden accepted that it is necessary to switch off emission control systems at low temperatures to protect engines. However, he seemed unconvinced that action is required at 18 Celsius. He said:

"If we were talking about higher emissions below zero, that would be more understandable and there are reasons why the engine needs to be protected. But what we've got is this odd situation where the temperature threshold has been set far too high. That is a surprise".

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